In an older era, Telonic and HP made Rho-tectors as I remember. The beauty of these was the components of the bridge were precision, that is the resistors were matched on one side and the diode compensated for linearity. No transformer. There were 4 ports. One port had the precision reference 50 termination, the opposite side had the DUT, the third port was RF, and the last port the DC signal output. The sweeper was fed into the RF port and the DC to the vertical channel of a scope, the swept voltage went to the horizontal channel. If handled properly they are good from 1 to 1500 MHz. It is easy to see 3 to 30 MHz for checking a cable. Depending on the sweeper, accuracy could be easily calibrated with 25, 50, 75, and 100 ohm precision loads. 1.5:1 would give you 2 cms of deflection. I have a home brew one and it is my SWR reference to check the portable measurement device. No it doesn't give you R and j, sorry.
Mel, K6KBE On Thursday, November 21, 2013 1:22 PM, ab2tc <ab...@arrl.net> wrote: Hi, I am not aware of any "Wattmeters/SWR Bridges" that work by reading E and estimating power as (E^2)/50 ohm. If it did, it would not be able to determine any kind of estimate of SWR or reflected power. All the "Wattmeters/SWR Bridges" I am aware of are constructed with a bridge circuit that is in principle capable of correctly displaying forward and reflected power even with a complex load impedance. But they do rely on the performance of at least one transformer, often a capacitive divider for sensing voltage and diode rectifiers with their inherent non-linearities, especially at the low power levels preferred for tune-ups. These components are often chosen for their lowest cost rather than performance and calibration may be sloppy. Even with quality RF components, unless the device has a microcontroller that can store and apply corrections for frequency, power level and temperature, accuracy is generally poor, easily exceeding 5-10% of full scale. High end devices that do much better than that are available but they come at a cost. Nothing in the instrument can compensate for the errors caused by common mode currents in the coax though so it's always important to keep the RF out of the shack. Three instruments in a line next to each other *should* read the same forward, reflected power and SWR under ideal conditions. AB2TC - Knut k6dgw wrote > Most ham-grade "Wattmeters/SWR Bridges" that I've encountered read E, > assume R, and "calculate" P in an analog fashion. When SWR is not 1:1, > Z is not equal to 50 ohms, and in most cases, the higher the SWR, the > higher the power indicated on the meter. For these instruments, nothing > you see on the indicator is true unless the SWR is 1:1 [i.e. the > impedance is 50 ohms]. Even then, many are sensitive to common-mode > current on the outside of the coax shield, and most of us have *some* > common-mode conduction on our feedlines. > > I once put three ham-grade "wattmeters" in series right next to each > other on 20m, and they all read differently. When I changed their order > in the feedline, they all read differently ... and different from what > they had before. I concluded that, unless I was willing to spend the > money to acquire test equipment that actually sensed and displayed watts > and not E^2/R, I'd have to be satisfied with approximations. > > 73, > > Fred K6DGW > - Northern California Contest Club > - CU in the 2014 Cal QSO Party 4-5 Oct 2014 > - www.cqp.org > <snip> -- View this message in context: http://elecraft.365791.n2.nabble.com/Internal-K3-SWR-vs-Extermal-SWR-Meter-tp7580938p7581035.html Sent from the Elecraft mailing list archive at Nabble.com. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
